1. Field of the Invention
The present invention relates to a display panel using liquid crystal or the like as a display medium, and more particularly, the present invention relates to a wiring pattern of the display panel in which slits and openings are formed to permit passage of light for irradiation of a photo-setting sealing material, as encountered in a case where a photo-setting sealing material is used to bond together a pair of transparent substrates in the fabrication process of such a display panel.
2. Description of the Related Art
What is sought after in display panels is high picture quality and high resolution, in addition to frame narrowing.
Various attempts have been made to achieve frame narrowing. According to one such attempt, in a display panel in which a photo-setting sealing material (hereinafter also simply referred to as “sealing material”) that hardens when irradiated with light such as ultraviolet rays is used to bond together a wiring substrate and a color filter substrate, attention is paid to the arrangement of the sealing material relative to a light-shielding film formed in the frame region on the color filter substrate.
Conventionally, after the bonding together of the two substrates, as seen in a plan view, the sealing material is arranged outside the light-shielding film. This is because the light-shielding film naturally shields light and therefore, placing the sealing material right below it makes irradiation of the sealing material with light impossible. Thus, the frame region needs to be at least as wide as, and hence not less wide than, the width of the light-shielding film plus the width of the region over which the sealing material is applied.
At this point, if openings are formed in the light-shielding film so that, when the two substrates are put together, those light passages are located right above the sealing material, then by shining light from directly above the light-shielding film, it is possible to harden the sealing material located directly below it. This method is disclosed in JP-A-2000-089235.
This method permits a photo-setting sealing material to be placed right below a light-shielding film, and thereby contributes to frame narrowing by the width of the sealing material region.
Such openings formed in a light-shielding film are typically given a parallel-wiring (lines and spaces) structure as shown in FIG. 4, or a mesh-like structure having a rectangular unit opening repeated vertically and horizontally as shown in FIGS. 5A and 5B.
In the first place, however, forming openings in a light-shielding film risks spoiling its essential function, namely the function of preventing unnecessary leakage of light from the frame region to the display region.
On the other hand, another method is known whereby, while a light-shielding film is left untouched, openings are formed in a wiring on the opposite substrate, namely the wiring substrate, so that a sealing material is hardened by being irradiated with light from behind the wiring substrate.
In this case, the size and shape of openings formed in a light-shielding film cannot be applied to openings formed in a wiring.
The reason is that, in a wiring, not to mention such wide openings having a parallel-wiring structure as would be formed in a light-shielding film as described above, even narrow openings such as slits produce electrical resistance. In a case where slits are formed in a wiring, from the perspective of minimizing electrical resistance, any portion of the wiring located between adjacent slits needs to be made as wide as possible.
In contrast, from the perspective of maximizing the amount of light (light energy) with which a photo-setting sealing material is irradiated, any portion of the wiring located between adjacent slits needs to be made as narrow as possible. This is because making such a portion of the wiring too wide may prevent the light diffracted around from the adjoining slits from reaching the width center of that portion, leaving the sealing material there unhardened.
As described above, in a case where slits or openings are formed in a wiring for the purpose of hardening a photo-setting sealing material, their size and shape need to be worked out with consideration given to the tradeoff between electrical resistance and how light reaches the sealing material.
Incidentally, today, as one way to achieve cost reduction in display panels, boardless wirings are often adopted. Boardless wirings are wirings arranged outside the display region (i.e., in the frame region) of a display panel, as a replacement for a shared board (PWB, printed wiring board) that is inserted between a display panel (typically, a liquid crystal display panel) and a signal control board to connect the driver connected to the former to the latter. This method of wiring eliminates the need for the shared board, and thus contributes to cost reduction. An example of beardless wirings is disclosed in, for example, JP-A-2001-056481.
In a set of boardless wirings, a plurality of wirings having different functions are arranged together. These wirings have different widths from one another, and in addition, their widths vary along their lengths, each wiring thus having an irregular pattern. This is schematically shown in FIG. 6A. In the figure, a part between the upper and lower horizontal two dot chain lines is where a sealing material is laid, and one of a set of boardless wirings crosses that part.
Suppose that, in this one of the set of beardless wirings, the above mentioned parallel-wiring structure is applied, and the wiring is separated from the right side into equally wide bands with slits formed between them. Then, as indicated by hatching in the left part of FIG. 6B, an odd region is left. At this point, the width of each of the equally wide bands thus separated with slits formed between them is optimized such that, when being irradiated with light having a predetermined energy, the light diffracted around both sides of each band can completely harden the photo-setting sealing material located at the top of that band, and in addition, such that the electrical resistance of the bands is so low as not to affect the driving of and display with liquid crystal.
Seemingly, giving this odd region a wiring figure pattern as shown in FIG. 6C is likely to permit the sealing material to be irradiated with light sufficiently. Now the wiring figure pattern shown in FIG. 6C will be studied for a while.
The odd region in the left is patterned such that, as it extends from the start point to the end point of the region where it overlaps with the sealing material, its width branches into two with a slit between them near the location where the width becomes about twice or more than the optimal width mentioned above for the first time. Of the resulting two, the first forms a band that remains equal to the optimal width from the branch point to its end point; the second, while extending to its end point along the first with a slit between them, further branches into two near the location where the width becomes about twice or more the optimal width mentioned above for the first time. In the figure, this branching is repeated once more before reaching the end point.
In this way, the odd region is separated into bands having the optimal width to form as many slits as permissible, in an attempt to increase the efficiency of the hardening of the sealing material over the entire wiring. Even then, near the branch points (in FIG. 6C, the parts encircled in closed curves indicated with the reference numeral 30), the width of the band is still larger than the optimal width; thus, there, the sealing material may be left unhardened. This is overcome in preferred embodiments of the present invention that will be presented later, and how that is achieved will be explained in the descriptions of the individual preferred embodiments that will be provided later.
Next, a description will be given of a case where mesh-like structure as shown in FIG. 5A or 5B is formed in a wiring. In FIG. 5A is shown an example of a pattern in which a mesh-like structure is formed with square unit openings 20 and the distance between adjacent openings 20 is equal to the length L of each side of the square. In this case, the distance L between adjacent openings, that is, the width L of each wiring portion separated by the openings 20, is optimized, as described above, such that the electrical resistance of the wiring portions is so low as not to affect the driving of and display with liquid crystal, and in addition that the light diffracted around both ends of the width can completely overlap at the center of the width to permit the sealing material right above it to harden completely.
In FIG. 5B is shown an example of a mesh-like structure that is formed with rectangular unit openings 21 measuring S along their shorter sides and L along their longer sides (S<L), with the unit openings 21 arrayed at a distance equal to the length L of their longer sides from one another both vertically and horizontally.
In this case also, the width of each wiring portion separated by the openings 21, that is, the length L of the longer sides of the openings 21, is optimized such that the electrical resistance of the wiring portions is so low as not to affect the driving of and display with liquid crystal, and in addition that the light diffracted around both ends of the width can completely overlap at the center of the width to permit the sealing material right above it to harden completely. Inconveniently, however, such mesh-like structures have the following disadvantages irrespective of whether or not they are applied to a beardless wiring.
In the above described mesh-like structure having square unit openings, the distance between diagonally adjacent squares, as measured between their mutually nearest vertices along the diagonal line, is √{square root over ( )}2L. Thus, the light diffracted around both ends of this distance does not overlap at the center of the √{square root over ( )}2L long line. Accordingly, the sealing material is left unhardened around the center of that line.
The same occurs with the rectangular openings 21 shown in FIG. 5B. In this case also, the distance between the mutually nearest vertices of diagonally adjacent openings 21 is √{square root over ( )}2L. Thus, as with the square openings 20, the sealing material may be left unhardened around the center of the √{square root over ( )}2L long line.
If there are many such unhardened parts scattered over the sealing material, they weaken the bond between substrates, and may even make it impossible to bond them at all in the worst case.
Moreover, if such unhardened parts of the sealing material are located at the boundary with a liquid crystal layer, the unhardened sealing material may flow into the liquid crystal layer, adversely affecting the display performance of the panel.